A roof apparatus for a vehicle that reduces an air resistance while the vehicle is being driven is disclosed in JP4161201B (which will be hereinafter referred to as Reference 1), for example. The roof apparatus disclosed in Reference 1 includes plural bumps (protrusions) at a rear portion on a roof of a vehicle. Each of the plural bumps has a height specified in consideration of a boundary layer of airflow flowing on the roof and includes a shape achieved by a half of a spheroid divided at a surface including a longitudinal axial line of the spheroid. The plural bumps are substantially arranged side by side in a width direction of the vehicle. In a case where the vehicle is driven, airflow flowing on the roof forms two pieces of longitudinal vortex at a rear side of the bumps because of the shapes of the bumps. A separation point of air is thus shifted to the rear side of the vehicle from around a boundary between an outer surface of the roof and an outer surface of a rear body of the vehicle. As a result, a pressure (an atmospheric pressure) in the rear of the rear body of the vehicle increases, i.e., a negative pressure is cancelled by atmospheric pressure recovery to thereby decrease a coefficient of air resistance (cd value) of the vehicle as a whole.
In addition, another roof apparatus for a vehicle is disclosed in JP04-8681A (which will be hereinafter referred to as Reference 2). The roof apparatus disclosed in Reference 2 includes fins arranged at left and right side portions of the vehicle so that each upper edge of the fins projects diagonally outwardly in the width direction of the vehicle. Each of the fins continuously extends along a boundary portion between a side surface and an upper surface of the vehicle from a roof end to a rear deck end in a longitudinal direction of the vehicle. A cross sectional shape of a connection portion between the side surface of the vehicle and the fin is substantially formed into an outward recess. Accordingly, airflow flowing over the upper edge of each of the fin from the side surface of the vehicle generates a vortex and a negative pressure at a leeward side of the fin to thereby press a rear portion of a vehicle body to the leeward side of the fin. As a result, a yawing moment caused by a cross wind is cancelled. A coefficient of lift decreases (i.e., a grip increases) and a driving stability of the vehicle increases accordingly.
Nevertheless, according to the roof apparatus disclosed in Reference 1, airflow flowing through the side surface of the vehicle is likely to join the boundary layer serving as a slow-speed airflow flowing in the vicinity of the outer surface of the roof between the most outward bump in the width direction of the vehicle and a roof ridge line of the roof at the most outward side in the width direction of the vehicle. Therefore, an effect to move the separation point of air to the rear side of the vehicle from the vicinity of the boundary layer may be reduced. A pressure increase at the rear side of the rear body of the vehicle is restrained, which may lead to a low fuel consumption.
In addition, according to the roof apparatus disclosed in Reference 2, airflow flowing from a fender (the side surface) to the rear deck (the rear body) by overriding the fin (i.e., the projecting member) generates the vortex at the leeward side of the fin. Thus, the leeward side of the fin tends to be the negative pressure to thereby push the rear portion of the vehicle to the leeward side (the rear portion of the vehicle is pulled to the leeward side). As a result, a forward driving force by an engine of the vehicle is reduced, which may lead to a low fuel consumption.
A need thus exists for a roof apparatus for a vehicle which is not susceptible to the drawback mentioned above.